Storage device having a photo-conductive target



A. D. COPE 3,136,909

STORAGE DEVICE HAVING A PHoTocoNDucTvE TARGET June 9, 1964 Filed July 10, 1959 l l l NN a 11 l I n posed on a transparent signal plate.

United States Patent Oii ice 3,136,909 STRAGE DEVCE HAVING A PHOTO- CONDUCTIVE TARGET f Appleton Danforth Cope, Hghtstown, NJ., assigner t0 Radio Corporation of America, a corporation of Delaware Filed July 10, 1959, Ser. No. 826,289 3 Claims. (Cl. 313-65) This invention relates to devices for storing electrostatic charges representative of an optical image. ln particular, this invention relates to photoconductive type devices for storing electrostatic charges.

l -lnthe prior art, there arel many known types of Astorage tubes which are capable of receiving an electrical pattern and storing, or retaining, this pattern for a long period of time, and then providing an electrical output signal corresponding to the stored pattern. There are also devicesin the prior art which receive an optical pattern, store this pattern for any desired length ot time, and then reproduce this pattern as a visible output image.

This invention diiers from these previously described devices in that this invention is directed to a storage device which receives an optical image, converts and stores this image as an'electrostatic charge for any desired length of time,` and reads out the image as an electrical output signal.

fOne typeof tube which receives an optical image, stores an electrostatic image for a short period of time, and then reads out the stored image, is -a pickup tube of the photoconductive type. The tubes of 'this type generally utilize a target in which a photoconductor is dis- These tubes are normally used as television camera tubes and, when used inthis manner; the photoconductive target accumulates or stores a charge throughout the period of time, say 1/30 of a second, between interrogations by the scanning electronrbeam. By selecting some of the more insulating type photoconductors for use in such a target, charges accumulated vbyeitposure of the p hotoconductor to an image can be usefully stored for periods of time considerably longer than l@ Vof asecond vbefore thev beamV reads out, the charges. The length of storage time availabley before the image deteriorates varies, within limits, with the resistance characteristics of the selected Photoconduct'o'r. Some'of the `more insulating photoconductive materials are capable of storing a charge pattern for periods-of the order oi one-half hour beforeserious image theearth, before the signal is read out and transmitted toY receivers; positioned on earth., yIn the rocket field, they problems of various `,otherfradiations affecting either the light sensitive properties, or the charge storage properties,

oi'any materialiselected mustbe considered. Theireaso'n or this is that thepickup and storage'device must pass device togcomplete its function.

. There are vavailable,certain pickup-storage( systems in the prior VIart'k which include a camera utubesensitiv'e to anl opticalg'image.- Theoutput of the camera tube is then applied to .aimagnetic taper or a storage tube for storage purposes.; rI 'hese systerrisl have the disadvantagefthat "the` originalimage mustibefre-imaged into the storage device,

thus losing considerable Vdeiinition of the stored picture.'

It"is, ther eiore,l an object of this inventiontofprovide a optically excited electrostatic storage device which is' capable of retaining an image for an indenite period y of time.

I l.through,,conce ntratedl radiation bands encircling theear'th v both before and after theimage is stored in order for the It is a still further object of this invention to provide a storage device which isi substantially unaffected by radiations other than those which are used to operate the device. I y I These and other objects are accomplished in accordance with this invention by providing a device including a signal plate, a photoconductor, and anginsulator; which are .in layers in the order-recited, vwith a means for producing an electrical charge pattern, corresponding to an optical image, between the ,signal plate and the exposed surface of the insulator. By proper selection of the mate.`

rials and geometry for the photoconductor-insulator sandwich, this charge pattern may be stored for any desired length of time, and may be read out in the form Of'an electrical signal. v A Y i The invention will be more clearly understood by 'reference to the following description when read in conjunction with the accompanying single Vsheet of ldrawings wherein:

FIG. 1 is a longitudinal and partly sectional View of device in accordance with this invention; 4

`FIG. 2 is a schematic representation of the operation ofthe target shown in FIG. 1; and, i

FIG. 3 is a partially schematic View of another embodiment of this invention. n ,Y v l Reference is now made to FIG. 1, which shows a camera type storage device 10 in accordance with this inverition. -The device 10 comprises 'an evacuated envelope 12v having an electron gun 16 in one end thereof. The elec# tron gun 16 may be any of the known' types lof electronV guns and produces an electron beam :directed toward aiV targetelectrode i8 in the other end ofthe envelope 12.

The electron beam is focused andjsca'nned over ytheje'x-` posedfsurface of the target electrode by 'any conventional means (not shown).

The target electrode 18comprises a transparent elec^ trical conductorl 20 having a layer, of 'pho'toconductivei material ZZ'thei-eon. The 'photo'c'onductor "22 is vin turn coveredby a layer of insulating material 24; The device 1,19 is similar to a Vidicon type camera tube except for the provision offthe insulation'layer 24 and theselection and geometry of the material used as 'the photocon'ductor 22. The photoconductor'ZZ, inaccordance with this'in! vention, is selected for its resistive 'characteristics and itssensitivity,fr`ather than the 'lack of 'photoconductivey lagA whichisl one 'of themajor factors'inselectingJniaterials for a commercial television camerattube.v

The transparent conducting signal plate 2l) maybe made of tin oxideV or a thin evaporated metaliilxi-r suchfas' gold. The sign'allplate` 2Q"may be deposited by 'fan'yl" known techniquefsuch as spraying or' evaporation. The signal plateV Ztlshould be', highly electrically,conductive andfshould be transpareirtto the particular radiant'nergyy fr'which the 'device 1:0 isv designedto respond. The photoconduc'tor ZZ is made Ltany one of niany known materials, ormixtures""`ther=.c' 'f,y which changesits rsistance in response" to radiant energyl directed thereon:

When the vdevice l'tlisdesigned so as'tovbe 'sensitive toa1 particular wav'elengthlof; radiant "energy, eig. visible lightl or infrared energy, the `pljiotoconduct'orisselected sothatVVV itssensitivity to the` selected wavelength is'high.` For the visible light range amorphous"aritinionytiiLsulphidelhasi been foundV suitable. Forinfrared "radiations 'a combinationfof PbSV and 'PbQ is suitable',- while for ultraviolety radiations either amorphous Se or livb"aresuitable.v TheY Patented June 9, 1964 photoconductor Y2.2. may be depositedr by evaporation for example.

The insulating layer 24 may be any known electrical insulator. The particular material selected, and its thickness, are determined by the resistance of the material and by the length of time during which the image is to be stored. In short, the geometryof the material is selected so that the dielectric relaxation time (RC` time constant) is substantially greater than the storage time desired. The insulating layer 24 and the photoconductor'22 are selected 'so that the combination, or the target 18, will have a resistivity that is greater than A1015 ohm-cm. As an example, with a photoconductive layer 22 of 5 microns of antimony tri-sulphide, a suitable insulaing layer would be a layer' of mica microns thick. This type of target has Vbeen found to produce storage, without serious picture deteri` orationjfor as long as several hours. For longer storage periods, e.g. 24 hours, higher'resistance insulating layers are chosen, e.g. polystyrene of other organic insulators,`

and/or thicker layers are deposited. In general, the resist'ivity of the target is determined by the resistivity of the insulator which inturn determines the storage time.V

The resistivity of the photoconducor is to be considered onlyin that this resistivity should be high enough so that,Y

f or scene areas less than the maximum brightness, the photoconductor does not become completely discharged during the exposure time.

e The insulating layer 24 may be deposited by well known techniques such, as spraying settling, evaporation, or sputtering. In the alternative, the insulating layer 24 may formV the support for thetar'get and have the other materials deposited thereon.. l

The operation of the device 10, with reference to FIG. 2 is as follows. With no light on the. target electrode,

and with thel potentials shown in FIG. 2 applied to the tube, the electronbeam scans the exposed surface of the Yinsulating material 24 and 'drives thescanned surface of the insulating layer 24 to the potential of the collector a charge image may be retained forperiods Yof days, as in accordance with this invention, the total target resistivity is greater than V15 ohm-cm.

To read out the charge image, the signal plate side of the insulating layer 24 is brought to a uniform potential, i.e. 150 v.u This may be done by connecting the signal plate to the requilibrium potential either by allowing sucient time forpdark leakage through the photoconductor, or `by shining a uniform light on the photoconf ductor. Whenoperating for long storage times, e.g. one hour or more, and when the dark current ow through the photoconductor having aresistivity of lessthan 101 Y ohm-cm., the leakage method is preferred since it is automatic; For photoconductors having a high resistivity, or

for short storage periods, a blankinglight should be di" rected onto the photoconductor toV -bring the signal'plate side ofthe insulator to the equilibrium potential. p Afterv either of these methods is used tok bring the interface to f in so doing, generates a signal in the signalplate by'ca'pacif mesh 26 (150 volts). Now, with the electron beam and 'i the optical imageot, the potential of the signal platev 20 is raised, e.g. to 300'volts. Under these conditions, the potential distribution across theV sandwich of the insulator and photoconductor is in accordance with the relative resistances of the two layers. This is representedfb'y curve 30 of FIG. 2. The target 18 is now lprepared for the storage of'an image. With the jelectron beam still off, and when a lightV or optical image' is directed ontothe photoconductor, the resistance ofeelementaliareas of the' photoconductor 'is reduced in 'proportionto the amount and shade of image focused onto these areas of the photoconductor. Thus, the potential drop' Vacross `the illuminated -area vof the photoconduetor is now substantially' reduced by currentV flow through the photoconductor.

With thesignal plate potential fixed, the potential of the w interface between photoconductor and insulatorrises by: an amountfdepending on the light-induced conductivity.i.A

The potential across the insulator is not changed.'` However, .dueto the thinness 'of' .theinsulator 24', and 'by capacitive couplingacross the insulator, the front surface lof the insulator. isV also raised 4in potentiallby an amount.

equallto the rise in potential `of the interface to point `$2.

If now the electron .beam isV turned on, the electron Y beam returns-the scanned surface' of the insulator to cole lector potential ('150V01ts) so that a ,potential distribue. `tion across theeinsulatorkg-Zl2 is developed which' is repre;vv

` ported so that it is lmovable .across a group of'opposed roller contacts 44 and 46. The movement may-beprog duced by thevrotationof storage reels(not shown)f1forl` storing the tape both before andafter kthe input informa-f l *D uringV the storagea xedpotential, the pattern of charge will'appear on the exposed face ofthe insulator. Areas of the insulator Y over a signal exposed' area of photoconductor are more negative, on the lscanned face of Vthe insulator, than the Y unexposed or dark elemental areas ofthe insulator.v .Un-V` der 'these conditions, and with the signal plate still connected to the equilibrium potential, Vthe electronbeam, when scanned over the target, restores the vscanned sur@ face of the insulator 24v tothe equilibrium potential fand,

tive coupling. The'signal generated is negative inpolarity as compared to the signal developed in standard Vidicon type operation.V The stored charge is removed bythe firstV few scans of the beam; then no furthersignal is available. untill the exposure cycle is repeated. The reading out of thestored charge primes or readies the device for thek Referring now to FIG. 3 there isrshown an embodiment of this invention which is designed for the storage of large 1 quantities ofinformation. The device shown in FIG. v3

comprises a ribbon or tapeof the photoconductor-insu'- lator sandwich 'including a layer of insulating'materia140 and also a layer of photoconductive.material.42.. The

materials `described in connection with FIG. l'may also be used in the device of FIG. 3 except that thel photoconductor-insulator sandwich should 'be strong enough to'be self-supportingand flexible enough to be capable-of Jbeingfr'olled. l f Y VVThe photoconductor-insulator sandwich ortaprelissur'a-V tion isstor'ed thereon. Y Y

.. insulator sandwich-.or ltape -is moved Yacrossthe roller-V sented'bycurve 34.v The` curve 34 Aindicateslthe condition j ofa highordei-of brightness Yof the ,scene during .thefexf' pos'ure time. Forscenes having less'biightness', the slope' 'of theline34isless. If the beam is 'now'turned off andL the lightjis turned off, the'fcharge vpattern existing. on both. j sides.er.theihsmatingflyef 24mm remamindenite1y-.: The factor whieh controls the period of time during .which contacts,therollercntact 44r functionsfaslthe signal plate; -k yin thejembo'diment "showndrr'EIGiffl While thegupper` f 1 roller-contact 46A functions :as the' electron beam-of fFIGi lVVV #for thejstorage or charging of tliesandwich.- 'L-Itfshould-jbef Yunderstood thatan electrn beam couldbe substituted Afori dimensional image.` The electronQbeamf48*couldlbejregj the charge will be stored is the insulating property'orthe dieIectricreIaXatiCn. time oftheiusulator. Asrisrwell known, certain insulators are highly insulating and thus,

the upper contact 746.," The electron-beamflffunctions asthe electronbeaxlof FIGLfl for the readout ofgatwo: r I Y p laced'with a rollerQ-icontact'for single :line ori singletrack:V l., read'outof'information;

The ldevice. V'shown inQ'FIGaV 3frnayYbeenclosedin'anfA evacuated container (not shown in FIG. 3) if electron beams are to be used and if the device is to operate on earth. Even if an electron beam is to be used, when the device is to be used for scientic rocket investigation purposes, no evacuated container is necessary because of the lack of atmosphere at the operating points or altitude. With only roller-contacts, and earth operation, no evacuated container is necessary.

Devices in accordance with this invention have been exposed to many radiations such as gamma rays, etc., both before and after charge storage has occurred, and have been found to operate as designed. Thus, both embodiments of this invention are believed to be useful particularly for scientific rocket investigation of remote planets.

What is claimed is:

1. A target for a storage device comprising a layer of insulating material in physical contact with a layer of photoconductive material, said layers of material having a resistivity of at least 1015 ohm-cm., and electrical means for scanning an exposed surface of said insulator.

with an electron beam, the resistivity of said target being at least 1015 ohm-cm.

References Cited in the file of this patent UNITED STATES PATENTS 2,588,292 Rittner et al. Mar. 4, 1952 2,603,757 Sheldon July 15, 1952 2,622,219 Schagen Dec. 16, 1952 2,687,484 Weimer Aug. 24, 1954 2,777,040 Kazan Ian. 8, 1957 2,843,773 Wardley Iuly 15, 1958 2,856,533 Rosenthal Oct. 14, 1958 2,900,569 Cope Aug. 18, 1959 2,901,348 Dessauer et al. Aug. 25, 1959 2,922,883 Giaimo Jan. 26, 1960 2,945,973 Anderson July 19, 1960 2,946,682 Lauriello July 26, 1960 FOREIGN PATENTS 763,745 Great Britain Dec. 19, 1956 

1. A TARGET FOR A STORAGE DEVICE COMPRISING A LAYER OF INSULATING MATERIAL IN PHYSICAL CONTACT WITH A LAYER OF PHOTOCONDUCTIVE MATERIAL, SAID LAYERS OF MATERIAL HAVING A RESISTIVITY OF AT LEAST 10**15 OHM-CM., AND ELECTRICAL MEANS FOR SCANNING AN EXPOSED SURFACE OF SAID INSULATOR. 